1. Field of the Invention
The present invention relates to an ultrasonic Doppler blood flow monitoring system, and more specifically to a blood flow monitoring system such that a catheter provided with ultrasonic transducers or vibrators is inserted into a blood vessel to transmit and receive an ultrasonic wave signal to and from the blood flowing through the blood vessel, in order to measure the velocity of flowing blood on the basis of the ultrasonic Doppler shift signals and further to calculate the amount of flowing blood.
2. Description of the Prior Art
With the advance of the medical technique, various serious diseases difficult in surgical treatment have become operable and therefore remediable recently. In the complicated operations performed for these diseases, there exists a tendency that a long operation time is required. Therefore, it is very important for the operator to monitor the cardiac functions of a patient accurately, that is, to grasp the condition of the patient at all times during a long-time operation.
At present, there is well known a method of obtaining cardiac outputs (the amount of blood fed out of a heart) in accordance with thermodilution measurement method, as the method of monitoring the cardiac function of a patient during and after operation. In this thermodilution measurement method, a catheter is inserted from the vein through the right atrium of the heart to the pulmonary artery; cool water is injected into the right atrium through the catheter; and the cardiac output can be estimated on the basis of the change in temperature in the pulmonary artery. This catheter is provided with a balloon at the distal tip thereof. Therefore, when this balloon is expanded, it is possible to smoothly carry the catheter from the right atrium to the capillary portions of the pulmonary artery on the blood flow. The above-mentioned catheter is called SWAN-GANZ catheter and is widely used. As far as no vascular contrast medium is injected, the catheter itself can be easily inserted into the blood vessel without involving any risk.
In the case of the above-mentioned thermodilution measurement method, however, since cooling water must be injected into the right atrium of the heart, the cardiac output is to be inspected at time intervals of about one hour at the most. As a result, there exists a problem in that it is impossible to monitor the cardiac output continuously for many hours. In other words, in the thermodilution measurement method, there exists a problem in that a sudden change of the condition of a patient under operation cannot be detected immediately and therefore appropriate countermeasures to be taken are delayed. The most important functions required by anesthetists, internists, and cardiac surgeons are to monitor the cardiac functions continuously for many hours. From this standpoint, the thermodilution measurement method is not the one which can satisfy their requirements.
To overcome the above-mentioned problems involved in the thermodilution measurement method, CARDIOMETORICS Corp. has developed a system called DOPCOM/FLOWCATH (Commodity name) which can monitor cardiac functions continuously. In this monitoring system, a catheter provided with ultrasonic Doppler transducers are incorporated. The catheter is inserted into a pulmonary artery, and the ultrasonic Doppler transducers provided on the catheter emit ultrasonic waves into the blood vessel and then receive echoes returned therefrom. On the basis of the received ultrasonic Doppler signals resulted from the echoes, the inner diameter of the pulmonary artery and the velocity of flowing blood are both measured, in order to calculate the amount of flowing blood, that is, the cardiac output, so that the cardiac functions can be monitored.
The prior art catheter adopted for the above-mentioned monitoring system will be described hereinbelow with reference to the attached drawings.
Figs. 1(A) and (B) show the essential portion of a catheter 80 for the monitoring system. The catheter 80 is provided with a first ultrasonic transducer (ultrasonic vibrator) 81, a second ultrasonic transducer 82, and a third ultrasonic transducer 83. The first ultrasonic transducer 81 is disposed in the catheter 80 so that an ultrasonic wave can be transmitted and received at a predetermined angle with respect to the blood flow direction(V). The second and third ultrasonic transducers 82 and 83 are disposed in the catheter 80 facing away from each other so that the ultrasonic waves can be transmitted and received in the radial direction of the catheter 80. In this prior art monitoring system, the velocity of flowing blood is obtained on the basis of only the ultrasonic Doppler signal obtained by an ultrasonic wave transmitted and received in the single direction. In more details, the velocity component (v.sub..alpha.) in the transmission and reception direction of the first ultrasonic wave is calculated on the basis of an ultrasonic Doppler shift frequency obtained by the first ultrasonic transducer 81; the calculated velocity component (v.sub..alpha.) is further corrected on the basis of the angle (.alpha.) of incidence of the ultrasonic wave with respect to the blood flow direction (V) to calculate an absolute value (v) of the velocity of flowing blood. In the above-mentioned calculation, the assumption is made that the catheter 80 is disposed in parallel to the blood flow direction and therefore the angle (.alpha.) of incidence of the ultrasonic wave is equal to the installation angle of the first ultrasonic transducer 81 relative to the catheter 80. The above-mentioned relationship can be represented as follows: EQU v=v.sub..alpha. /cos .alpha.
Further, the second and third ultrasonic transducers 82 and 83 also transmit and receive ultrasonic waves to measure the two radial distances d.sub.1 and d.sub.2 to the wall 91 of the blood vessel. The blood vessel diameter (D) can be expressed by an addition of the distances d.sub.1 and d.sub.2 and a radial distance interval de between the second and third transducers 82 and 83 as follows: EQU D=d.sub.1 +d.sub.2 +d.sub.0
On the basis of the blood vessel diameter D, the cross sectional area of the blood vessel can be calculated as EQU A=.pi.D.sup.2 /4
On the basis of both the cross-sectional area (A) and the velocity (v) of flowing blood, the amount of flowing blood (Q) can be calculated as follows: EQU Q=v.times.A
As described above, in the DOPCOM/FLOWCATH system, the velocity of flowing blood is calculated on the assumption that the orientation of the catheter 80 is in parallel to the blood flow direction (V), and further the amount of flowing blood is calculated on the assumption that the sum of the measured radial distances d.sub.1 and d.sub.2 between each transducer and the blood vessel wall and the distance d.sub.0 between the transducers are roughly equal to the blood vessel diameter (D). However, in a case where the catheter 80 is placed in a bent portion of the blood vessel, the orientation of the catheter is not always in parallel to the blood flow direction (V), as depicted in FIG. 2. In addition, in a case where the catheter 80 is placed in the blood vessel with being offset from the center of the blood vessel as depicted in FIG. 3, the second and third transducers 82 and 83 cannot necessarily measure the blood vessel diameter accurately. Further, the cross-sectional shape of the blood vessel is not always to be circular.
As described above, in the prior art monitoring system, when the orientation of the catheter 80 is not in parallel to the blood flow direction (V), the angle (.alpha.) of incidence of the ultrasonic wave signal is not constant. As a result, the velocity (v) of the flowing blood calculated on the basis of the above values inevitably involves an error. In addition, when the catheter 80 is disposed with being offset from the center of the blood vessel, since the measured inner diameter of the blood vessel becomes a value (D1) smaller than the actual inner diameter (D) as depicted in FIG. 3, the cross-sectional area (A) calculated on the basis of the measured inner diameter of the blood vessel inevitably involves another error. Accordingly, the amount of flowing blood (Q) calculated as a product of the velocity (v) of the flowing blood and the cross-sectional area (A) of the blood vessel also inevitably includes an error.
Further, a major part of the clinical data so far obtained is dependent upon the thermodilution measurement method, and the clinical data obtained by the DOPCOM/FLOWCATH system are still few. Therefore, it is not sufficiently reliable to use the DOPCOM/FLOWCATH system for the actual operation.
In summary, the prior art method of measuring the amount of flowing blood by use of the DOPCOM/FLOWCATH system involves the following problems:
(1) Since the calculated velocity of flowing blood and the calculated cross-sectional diameter of the blood vessel are likely to include errors, respectively, the amount of flowing blood calculated on the basis of these values is not sufficiently accurate. PA1 (2) The clinical data obtained so far by this system is few, and therefore the clinical data enough to monitor the cardiac functions during operation are not yet accumulated.